Add DgElement collection component and CreateElementCollection action

[nilsdeppe]

Proposed changes

This is the last bit of code for GTS nodegroup with no functions of time. I'll submit another PR soon to add support for functions of time. I still need to finish support for LTS.

Upgrade instructions

Code review checklist

• [ ] The code is documented and the documentation renders correctly. Run make doc to generate the documentation locally into BUILD_DIR/docs/html. Then open index.html.
• [ ] The code follows the stylistic and code quality guidelines listed in the code review guide.
• [ ] The PR lists upgrade instructions and is labeled bugfix or new feature if appropriate.

Further comments